Water treatment system for removing hexavalent chromium

ABSTRACT

A method for removing chromium from a water source can include delivering source water having a hexavalent chromium oxyanion into contact with an ion exchange resin to exchange the hexavalent chromium oxyanion for an anion and convert the source water to treated water, discharging the treated water, rinsing the ion exchange resin with a brine solution to remove the hexavalent chromium oxyanion from the ion exchange resin, converting the hexavalent chromium oxyanion to a trivalent chromium cation, binding the trivalent chromium cation to a chelating resin, and removing the trivalent chromium cation bound to the chelating resin.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/170,173, filed Jun. 1, 2016, entitled “WATER TREATMENT SYSTEM ANDMETHOD” and also claims priority to U.S. Provisional Patent ApplicationNo. 62/172,017, filed Jun. 5, 2015, entitled “WATER TREATMENT SYSTEM ANDMETHOD”, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

This disclosure relates to the treatment of water and, morespecifically, to removal of chromium from water.

BACKGROUND

Chromium is a naturally occurring metal found in rock, soil, atmosphericgas, and biological organisms. Chromium is found in two predominantvalence states: trivalent (Cr(III)) and hexavalent (Cr(VI)). Thepresence of chromium, particularly hexavalent chromium, in water sourcesis becoming an increasing health problem and an environmental problemwith industrial wastewater effluent streams. Hexavalent chromium occursin water sources as pH dependent oxyanion species. Typical oxyanionspecies include HCrO₄ ⁻¹, CrO₄ ⁻², and Cr₂O₇ ⁻². Existing methods ofremoving chromium from water produce a significant volume of wasteresiduals including, in some cases, hazardous waste.

SUMMARY

In general, this disclosure is directed to systems and methods for watertreatment to remove chromium, such as hexavalent chromium, from water.In certain embodiments, hexavalent chromium oxyanions in source waterare captured and converted to trivalent chromium cations which are inturn captured and discharged. Accordingly, embodiments of the disclosureare useful for creating treated water with lower levels of hexavalentchromium oxyanions.

In one embodiment, a method for removing chromium from a water sourceincludes delivering source water having a hexavalent chromium oxyanioninto contact with an ion exchange resin to exchange the hexavalentchromium oxyanion for an anion and convert the source water to treatedwater, discharging the treated water, rinsing the ion exchange resinwith a brine solution to remove the hexavalent chromium oxyanion fromthe ion exchange resin, converting the hexavalent chromium oxyanion to atrivalent chromium cation, binding the trivalent chromium cation to achelating resin, and removing the trivalent chromium cation bound to thechelating resin.

In another embodiment, a method for removing chromium from a watersource includes delivering source water having a hexavalent chromiumoxyanion into contact with an ion exchange resin to exchange thehexavalent chromium oxyanion for an anion and convert the source waterto treated water, discharging the treated water, rinsing the ionexchange resin with a brine solution to remove the hexavalent chromiumoxyanion from the ion exchange resin and regenerate the ion exchangeresin, adjusting the pH of the brine solution prior to converting thehexavalent chromium oxyanion to a trivalent chromium cation, deliveringthe brine solution into contact with a reducing agent to convert thehexavalent chromium oxyanion to the trivalent chromium cation,delivering the brine solution into contact with a chelating resin tobind the trivalent chromium cation to the chelating resin, and removingthe chelating resin with the trivalent chromium cation bound to thechelating resin.

In another embodiment, a system for removing chromium from a watersource includes a first vessel containing an ion exchange resin. Thefirst vessel is configured to receive source water and exchange ahexavalent chromium oxyanion for an anion and convert the source waterto treated water, and is configured to receive a brine solution toremove the hexavalent chromium oxyanion from the ion exchange resin. Thesystem also includes a second vessel for receiving the brine solutionwith the hexavalent chromium oxyanion from the first vessel, a thirdvessel containing a reducing agent for converting the hexavalentchromium oxyanion in the brine solution to a trivalent chromium cation,and a fourth vessel containing a chelating resin for binding thetrivalent chromium cation in the brine solution. The fourth vessel isconfigured to discharge the trivalent chromium cation bound to thechelating resin.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow diagram illustrating an exemplary method in accordancewith an embodiment of the disclosure.

FIG. 2 is a schematic diagram illustrating an exemplary system inaccordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

The following detailed description is exemplary in nature and is notintended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the following description provides somepractical illustrations for implementing examples of the presentinvention. Examples of constructions, materials, dimensions, andmanufacturing processes are provided for selected elements, and allother elements employ that which is known to those of ordinary skill inthe field of the invention. Those skilled in the art will recognize thatmany of the noted examples have a variety of suitable alternatives.

Embodiments of the disclosure include methods and systems for removingchromium from a water source. An embodiment of a method in accordancewith the disclosure includes reducing the level of hexavalent chromiumoxyanions (e.g. HCrO₄ ⁻¹, CrO₄ ⁻², and Cr₂O₇ ⁻²) in a water source bycontacting the water source with an ion exchange resin.

FIG. 1 is a flow diagram illustrating an exemplary method in accordancewith an embodiment of the disclosure. As shown in FIG. 1, in certainembodiments, the method includes delivering source water havinghexavalent chromium oxyanions into contact with an ion exchange resin inorder to exchange the hexavalent chromium oxyanions with other anionsand convert the source water to treated water (101). The method can alsoinclude discharging the treated water (102). In some embodiments, thetreated water can be discharged for direct use in a downstream processor to a holding tank or sewer. The method can also include rinsing theion exchange resin (103). In some embodiments, the ion exchange resincan be rinsed with a brine solution to remove the hexavalent chromiumoxyanion from the ion exchange resin. The method can also includeconverting the hexavalent chromium oxyanions to trivalent chromiumcations (104). In other embodiments, the method of FIG. 1 can alsoinclude binding the trivalent chromium cations to a chelating ionexchange resin (105) and removing the trivalent chromium cations boundto the chelating resin (106). Note the order of the steps shown in FIG.1 is only exemplary, and steps may be performed in other orders orsimultaneously.

In treating the source water (101), any ion exchange resin useful forcapturing hexavalent chromium oxyanions and exchanging the hexavalentchromium oxyanions for another anion can be used. When source water ispassed through an ion exchange resin, hexavalent chromium oxyanions inthe source water are exchanged for the anions in the ion exchange resin.In some embodiments, the ion exchange resin is a basic anionic ionexchange resin, such as a strongly basic anionic exchange resin. Inother embodiments, the resin can include a cross-linked co-polymer, forexample, in the form of a plurality of beads. The cross-linkedco-polymer can be selected from the group consisting of styrenic,acrylic, and phenolic polymers. Any functional group selective forhexavalent chromium oxyanions can be utilized. In some embodiments, theion exchange resin can include a quaternary amine functional group, suchas a quaternary amine functional group selected from the groupconsisting of di-methyl, tri-methyl, tri-ethyl, tri-propyl, andtri-butyl amine. In certain embodiments, the anion resin can be in asulfate (SO₄ ⁻²) or a chloride (Cl) form.

As more and more source water is passed through the ion exchange resin,the ion exchange resin eventually becomes “exhausted” where the resincannot remove any more hexavalent chromium oxyanions. The exhaustedresin can be regenerated by rinsing the resin with a brine solution. Inrinsing the ion exchange resin (103), the brine solution can be used toremove the hexavalent chromium oxyanions from the ion exchange resin andregenerate the resin so that it can be used to treat additional sourcewater. The brine solution can be passed through the resin bed containingthe hexavalent chromium in a direct or counter flow direction, alongwith optional rinse water flushes. In some embodiments, the brinesolution includes water and a salt, such as sodium sulfate, potassiumsulfate, or magnesium sulfate. In certain embodiments, the brinesolution includes water with a concentration of salt of at least 10% byweight (e.g., between 10% and 20% by weight). In certain embodiments,the brine solution delivered to the ion exchange resin is basic. Forexample, the brine solution may have a pH of between 10 and 12, and morepreferably a pH of between 10.5 and 11.

In some embodiments, the pH of the brine solution carrying thehexavalent chromium oxyanion is adjusted prior to further treatment(e.g., prior to the conversion of the hexavalent chromium oxyanion to atrivalent chromium cation (104)). For example, the pH may be adjusted tobe more acidic, preferably to a pH of between 2.0 and 4.0. In a specificexample, the pH is adjusted to between 3.0 and 3.5. Having the pH atthis level can be useful for keeping the trivalent chromium cation insolution after it is created by conversion from the hexavalent chromiumoxyanion form. The pH of the brine solution can be adjusted by anymethod, such as by the addition of an acid (e.g., an acid selected fromthe group consisting of sulfuric acid and hydrochloric acid).

The hexavalent chromium oxyanions in the acidic brine solution can thenbe converted to the trivalent chromium cation form (104). In someembodiments, the brine solution carrying the hexavalent chromiumoxyanions can be contacted with a reducing agent to convert thehexavalent chromium oxyanions to trivalent chromium cations. In certainembodiments, the reducing agent includes granular activated carbon(GAC). The hexavalent chromium oxyanions can be reduced by the GAC totrivalent chromium cations while a stoichiometric equivalent of carbonis oxidized to carbon dioxide, such as by the following representativechemical equation:4HCrO₄ ⁻¹+3C+16H⁺→4Cr⁺³+3CO₂+10H2O

The trivalent chromium oxyanion can then be bound to a chelating ionexchange resin (105), so that it can be removed from the system anddiscarded as waste. In some embodiments, the brine solution (e.g.,acidic brine solution) carrying the trivalent chromium cations can bedelivered into contact with a chelating ion exchange resin to bind thetrivalent chromium cation to the chelating resin. The chelating resincan include any functional group useful for binding trivalent chromium,such as a functional group selected from the group consisting ofiminodiacetic, aminophosphonate, thiouronium, and thiol. The trivalentchromium cations strongly bind to the chelating resin, due to the highselective capacity of the chelating resin over the sodium cation in thebrine. The combination of the strong affinity for trivalent chromiumcations and high loading capacity of the resin allows for the productionof a low-volume of non-hazardous waste in a stable, bound form. This isadvantageous, because existing methods of removing chromium from watercan result in significant volumes of often hazardous waste.

In some embodiments, the method of FIG. 1 is a continuous process. Inother embodiments, the method is a batch process. In certainembodiments, the method can be performed as a continuous process betweenbatch process steps. For example, source water can be passed through theion exchange resin until the resin reaches a load threshold ofhexavalent chromium oxyanions. The source water flow can then bestopped, and the brine solution flow started to remove the hexavalentchromium oxyanions from the ion exchange resin and regenerate the resin.Upon hexavalent chromium removal and regeneration, the brine solutionflow can be stopped and the source water flow restarted. In a specificembodiment, the process can be run in this manner until the chelatingresin is fully bound to the trivalent chromium cation, at which pointflow can be stopped and the chelating resin with the bound trivalentchromium cation is removed and replaced. Parallel systems and/orredundant subsystems can be utilized to provide a continuous process todeliver a continuous supply of treated water.

Accordingly, methods and systems in accordance with the presentdisclosure are useful for removing chromium, such as hexavalentchromium, from source water to create treated water with acceptablelevels of chromium. In some embodiments, the treated water resultingfrom an embodiment of the method of FIG. 1 includes less than 10micrograms per liter of hexavalent chromium (or a hexavalent chromiumoxyanions thereof). In certain embodiments, the treated water includesless than 50 micrograms per liter of hexavalent chromium and trivalentchromium combined (or a hexavalent chromium oxyanions or trivalentchromium cations thereof).

Some embodiments of the method of FIG. 1 are also useful for passingother impurities in the source water through to the treated water, suchthat the impurities are not concentrated within the system. For example,in some embodiments, a substantial amount of any sulfates, nitrates, andarsenic in the source water is discharged with the treated water. In aspecific example, more than 70% (e.g., more than 80%) of any sulfates,nitrates, and arsenic in the source water is discharged with the treatedwater.

Certain embodiments recycle various streams to further conserve processinputs and reduce liquid waste. For example, some embodiments recyclethe brine solution to rinse the ion exchange bed after contact with thechelating resin. In certain embodiments, such as the embodiments wherethe pH of the brine solution has been adjusted to be more acidic, the pHof the brine solution may be adjusted to be more basic after the removalof the trivalent chromium. In a specific example, the pH is adjusted tobetween 10.5 and 12 (e.g. between 10.5 and 11). The pH of the brinesolution can be adjusted by any method, such as by the addition of abase (e.g., a caustic soda such as sodium hydroxide).

In some embodiments, a portion of the brine solution (e.g., dilute brinesolution) is diverted after contact with the ion exchange resin. Suchdiverted brine solution can be concentrated, such as by passing itthrough a system in which the brine is removed (e.g., a reverse osmosissystem). The concentrated brine can be recombined with the brinesolution stream in the system to be further processed as describedherein, and the liquid can be discharged.

FIG. 2 depicts an exemplary system in accordance with an embodiment ofthe disclosure. In the embodiment of FIG. 2, source water containinghexavalent chromium is delivered to a first vessel 201, which containsan ion exchange resin. The source water contacts the ion exchange resinin first vessel 201 to exchange the hexavalent chromium oxyanion foranother anion and convert the source water to treated water. As shown inFIG. 2, treated water can be discharged from the first vessel 201. Inthe embodiment of FIG. 2, brine solution can also be delivered to thefirst vessel 201 to remove the hexavalent chromium oxyanions from theion exchange resin and regenerate the resin. In some embodiments, rinsewater can also be delivered to the first vessel 201 to assist in rinsingthe ion exchange resin to regenerate the resin. In the embodiment shown,the spent brine solution can be delivered to a second vessel 202, wherethe pH of the spent brine solution can be adjusted. The spent brinesolution can then be delivered to a third vessel 203 containing areducing agent (e.g., GAC) to convert the hexavalent chromium oxyanionsto trivalent chromium cations and on to a fourth vessel 204 containing achelating resin to bind the trivalent chromium cations. The chelatingresin with the bound trivalent chromium cations can be removed from thefourth vessel 204. In the embodiment of FIG. 2, the brine solution isdelivered to a fifth vessel 205, where the pH of the brine solution canbe adjusted before the brine solution is recycled back to the firstvessel 201.

Also as shown in FIG. 2, a sixth vessel 206 and a seventh vessel 207 canbe provided to collect a portion of the diluted brine solution streamand re-concentrate it. Diluted brine can be collected in the sixthvessel 206 and circulated through the seventh vessel 207. The seventhvessel 207 can include a system, such as a reverse osmosis system, toseparate the brine elements from the clean liquid. The rejected brinecan be recirculated to the sixth vessel 206 until the brine in the sixthvessel 206 is sufficiently concentrated (e.g., at least about 10%) to bedelivered to the second vessel 202 to be further processed as discussedherein. The liquid permeate from the seventh vessel 207 can bedischarged from the system.

Each of the vessels referred to herein can be any suitable size or formaccording to its function, and include columns, reaction tanks, andholding tanks as needed. Pipes, valves, and fittings can be used toprovide fluid communication between the vessels. In some embodiments,parallel systems and/or subsystems can be utilized to provide forcontinuous source water treatment and treated water discharge.

Accordingly, some embodiments of the disclosure allow for the removal ofchromium from source water. In certain embodiments discussed herein,such removal is accomplished without any chemical addition to thetreated water discharged from the system. Further, embodiments includingthe recycled brine solution provide a significant reduction in liquidwaste. In addition, embodiments of the disclosure provide for solidwaste residuals that are relatively small in volume and have anon-hazardous classification.

Various examples have been described. These and other examples arewithin the scope of the following claims.

The invention claimed is:
 1. A system comprising: a first vessel containing an ion exchange resin, the first vessel configured to receive source water and exchange an hexavalent chromium oxyanion for an anion and convert the source water to treated water, and configured to receive a brine solution to remove the hexavalent chromium oxyanion from the ion exchange resin; a second vessel for receiving the brine solution with the hexavalent chromium oxyanion from the first vessel; a third vessel containing a reducing agent for converting the hexavalent chromium oxyanion in the brine solution to a trivalent chromium cation; and a fourth vessel containing a chelating resin for binding the trivalent chromium cation in the brine solution, the fourth vessel configured to discharge the trivalent chromium cation bound to the chelating resin.
 2. The system of claim 1, wherein the second vessel is configured to adjust the pH of the brine solution.
 3. The system of claim 1, further comprising: a fifth vessel for receiving the brine solution from the fourth vessel, adjusting the pH of the brine solution, and recycling the brine solution to the first vessel.
 4. The system of the claim 1, further comprising: a sixth vessel and a seventh vessel configured to re-concentrate the brine solution with the hexavalent chromium from the first vessel; wherein the sixth vessel is configured to receive the brine solution from the first vessel, cycle the brine solution through the seventh vessel, and deliver a concentrated brine solution to the second vessel; and wherein the seventh vessel contains a reverse osmosis system for producing the concentrated brine solution.
 5. The system of claim 1, wherein the ion exchange resin is a basic anionic ion exchange resin.
 6. The system of claim 1, wherein the ion exchange resin includes at least one of an acrylic, a styrenic, or a phenolic cross-linked co-polymer.
 7. The system of claim 1, wherein the ion exchange resin includes a quaternary amine functional group.
 8. The system of claim 1, wherein the anion is selected from the group consisting of a sulfate and a chloride.
 9. The system of claim 1, wherein the brine solution includes a salt selected from the group consisting of sodium sulfate, potassium sulfate, and magnesium sulfate.
 10. The system of claim 1, wherein the reducing agent includes a granular activated carbon.
 11. The system of claim 1, wherein the chelating resin includes a functional group selected from the group consisting of iminodiacetic, aminophosphonate, thiouronium, and thiol. 